The question would be, do you mean naturally occurring substance, or designer drug?

If designer drug, as "AMP citrate" or more correctly 1,3-dimethylbutylamine is (I find no credible evidence of it being naturally occurring) then there are multiple choices none of which are legal.

If naturally occurring, there is so far as I know nothing in the amphetamine-like category that both exists naturally and isn't already banned, but there are some excellent natural stimulants that aren't yet on the market.

I don't know what the next designer stims might be. The people that do this do not constrain themselves with any requirements that could narrow it down for me such as legality or safety. Pretty much the only thing is the Controlled Substances Act, and even there there's been at least one distributor who had been offering a Schedule IV substance (lorcaresin.) So it's pretty wide open.

Sometimes it's guessable though, such as that they could think "Well dimethylamylamine isn't being allowed anymore, so let's try dimethylbutylamine" (so-called "AMP citrate.")